The heat pump is a device which operates with equipment developed for mechanical refrigeration, but by designing the cycle to be reversible, the operation of the heat pump can provide heat for warmth when ambient temperatures are cold as well as normal cooling when ambient temperatures are hot.
There are two common types of heat pumps. The first is the vapor compression heat pump, and the second is the absorption heat pump. The vapor compression heat pump is by far the most prevalent type currently being used. It is characterized by a compressor, two heat exchangers, one termed a condenser and the other termed an evaporator, and an expansion valve or capillary tube. The power to the compressor in most instances is supplied by an electric motor. This type of heat pump is not much more expensive than a central air conditioning unit. Its efficiency, or coefficient of performance (defined as the ratio of the energy provided for heating or cooling to the energy required to power the system) is between 2.0 and 3.0 when ambient temperatures are greater than 15.degree. F. Therefore when electrical costs are 4.5 cents per kilowatt-hour or less, this type of heat pump is an economical heat source compared to heating with oil at 50 cents per gallon.
While the vapor compression heat pump has found a wide range of usage because of its economic advantages in moderate climates, the usage of the absorption-type heat pump has been limited.
The absorption heat pump is characterized by an absorber, a generator, several heat exchangers, one of which is termed a condenser and another termed an evaporator, and an expansion valve or capillary tube. The power to operate the absorption heat pump is a high temperature heat source capable of causing the refrigerant, a volatile species to separate from an absorbing solution in the generator. If the high temperature is achieved by burning a commercial fuel, it would appear that the absorption heat pump would be economically attractive for heating in comparison to an ordinary furnace.
The absorption heat pump however has not become commercially viable because its coefficient of performance is too low, approximately 1.40, and the unit's cost is very high. Several patents and other publications relating to this subject have suggested improvements to increase the coefficient of performance. However, early patents concerning the absorption heat pump show that the original concepts produce coefficients of performance near the Rankine efficiency limit. Therefore improved heat transfer arrangements have tended to require increased equipment costs that are not sufficiently offset by an increased efficiency. The absorption heat pump has consequently remained generally unsuitable for commercial use.